It is known that aircraft motoreactors generate substantial acoustic emissions against which steps must be taken with greatest effectiveness and as to which many improvements have been provided.
As in all technological advances, the greatest gains are quickly obtained with the current means that are well known, but to improve further the struggle against acoustic emissions, it is necessary to work very pointedly on combinations of materials, as in the present invention, the results obtained being significantly improved.
There will be selected in the description that follows, the example of walls of nacelles of aircraft motoreactors, because the explanations can be immediately comprehensible, but nevertheless the uses are very numerous in aeronautics, as well as in other fields such as gas turbines, heat engines or blowers and more generally all machines which generate substantial noise which should be damped under difficult conditions of temperature, pressure and/or mechanical resistance.
To damp noise, particularly through the walls, resonators of the Helmholtz type are used which permit attenuating in a reactive manner certain radial acoustic components under certain conditions of dimensioning of the material. Such a resonator comprises a hollow structure of the honeycomb type disposed between two resistive layers.
The honeycomb structure provides a cavity which permits attenuating by trapping certain noisy frequencies in a reactive manner.
The acoustically resistive layer has, in addition to its role of partitioning the hollow structure, a dissipating role, which is to say that it permits transforming acoustic energy into heat.
The present invention relates more particularly to the production of an acoustically resistive layer which permits obtaining physical attenuation by transformation of the acoustic energy into heat, particularly incident waves.
There are already known embodiments of such resistive layers made by combining a honeycomb structure and a total rear reflector.
A first example consists in using as the resistive layer perforated metal or composite sheet, which permits obtaining a single layer, high structural resistance and good control of the proportion of open surfaces.
On the other hand, this type of layer has high acoustical non-linearity, high dependence on tangential flow and low resistance to erosion in the case of a composite layer.
A second example is the combination of perforated metal sheet with metallic cloth or composite. In this case, there is achieved control of the porosity of the constituents and the possible adjustment of the proportion as well as the high structural resistance with supplemental advantages of moderated acoustic non-linearity and moderated dependence on flow.
By contrast, the layer is doubled, which requires a delicate assembly process, which is long and costly, with risks of acoustic inhomogeneity if this assembly has disparities, as well as the risk of corrosion. It should also be noted that the choice of the materials can be imposed by the requirements of assembly.
A third example of the prior art consists in combining a grill and a metallic cloth or composite.
In this case, the structural resistance is high and the phenomena of acoustic non-linearity and dependence on flow are moderated.
On the other hand, surface acoustic homogeneity is lacking, with risk of aerodynamic relief. Repeatability is difficult to obtain and the adjustment of the open proportion of the surface of the grill is delicate because there is a dispersion during fabrication and above all because of the unavailability of grills with adjustable surface area.
There can also be cited a fourth example which consists in using a metallic or synthetic cloth without structural reinforcement.
In this embodiment, there is a monolithic layer, low non-linearity, low dependence on tangential flow and good control of the proportion of porosity.
On the other hand, the structural resistance is unfortunately low, more particularly with cloths which have good properties of acoustic damping.